This invention relates to test devices, generally referred to as phantoms. More specifically, this invention relates to a phantom useful with nuclear magnetic resonance (NMR) scanner apparatus to carry out performance and calibration measurements in three dimensions without repositioning the phantom.
A phantom generally comprises a test object constructed to simulate structures and conditions encountered in actual use. In the case of medical diagnostic equipment, the phantom can be made to simulate various types of tissue and can be used as a substitute test object in operator training, as well as a calibration device to ascertain the level of equipment performance. In some cases, it is desirable to ascertain the degree of equipment operability by daily calibration procedures. The phantom, therefore, must be constructed to allow evaluation of multiple image quality parameters with relative ease and a minimum expenditure of operator time and effort. Accordingly, factors such as scan time to acquire the test data, phantom set-up time, phantom weight, and cost must be minimized. Conversely, factors such as realiability, repeatability and simplicity must be maximized.
Phantoms have been utilized in the past in such diagnostic modalities as transmission computed tomography (CT) and digital radiography. Phantoms for use with NMR apparatus, however, must meet different performance requirements than those of other modalities. This is due, at least in part, to the fact that NMR scanner operation is different from other modalities in that it is capable of detecting tissue parameters which are not measureable by any other means. Additionally, NMR has significantly longer imaging times, of the order of five minutes, than the afore-mentioned modalities, so that the need to optimize phantom performance is apparent. To better appreciate the unique requirements associated with NMR phantoms, it is beneficial to consider some fundamental NMR scanning principles.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons or neutrons. Due to the spin of the protons and the neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample including such nuclei is placed in a static, homogeneous magnetic field, B.sub.o, a greater number of nuclear magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the B.sub.o magnetic field, the magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency .omega., also referred to as the Larmor frequency, is given by the Lamor equation .omega.=.gamma.B, in which .gamma. is the gyromagnetic ratio which is constant for each NMR isotope and wherein B is the magnetic field acting upon the nuclear spins. It will be thus apparent that the precession frequency is dependent on the strength of the magnetic field in which the sample is positioned.
In order to observe an NMR signal, the orientation of magnetization M, normally directed along the magnetic field B.sub.o, must be perturbed by the application of a magnetic field oscillating at the Larmor frequency so as to create a transverse magnetization component in a plane orthogonal to the field B.sub.o. This is accomplished by applying a magnetic field, designated B.sub.1, in a plane orthogonal to the direction of the static field B.sub.o by means of radio frequency (RF) excitation pulses through coils connected to RF transmitting apparatus. The effect of field B.sub.1 is to rotate magnetization M in the volume of the object being studied which lies in the field of the RF coil. When the RF excitation is removed, magnetization M returns to its equilibrium position by a variety of processes and in the course of doing so, generates a detectable NMR signal.
While it is adequate for some purposes to simply detect the NMR signal originating from the entire volume lying within the field of the coil, it is frequently necessary to identify spatially where in the volume the NMR signal originates. One such application is in NMR imaging. Spatial localization is achieved by the application of the G.sub.x, G.sub.y and G.sub.z magnetic-field gradients directed along the x, y, and z axes of the conventional Cartesian coordinate system. The gradients are generally of the form EQU G.sub.x (t)=.differential.B.sub.o /.differential.x EQU G.sub.y (t)=.differential.B.sub.o /.differential.y EQU G.sub.z (t)=.differential.B.sub.o /.differential.z
The G.sub.x, G.sub.y, and G.sub.z gradients are constant throughout the imaging volume, but their magnitudes are typically time dependent. The gradients are utilized with radio frequency excitation pulses in various imaging techniques, such as those conventionally referred to as multiple-angle-projection reconstruction, and spin warp to acquire spatially resolvable NMR information.
A refinement of the technique to localize the NMR signal to a particular volume of interest (such as a slice, for example), rather than the sample volume lying within the field of the RF coil, is to utilize RF excitation pulses which are modulated to have a predetermined frequency content. Such RF pulses applied in the presence of magnetic field gradients are effective in exciting nuclear spins situated in preselected regions of the sample having resonant frequencies as predicted by the afore-described larmor equation. Radio frequency pulses modulated in this manner are referred to as being "selective." These should be contrasted to non-selective RF pulses which are applied in the absence of magnetic field gradients, as disclosed hereinbefore, and which affect all of the nuclear spins in the field of the coil.
It will be therefore recognized that judicious choice of RF and gradient pulses permits NMR information to be acquired from any preselected plane within the object. Typically, it is possible to collect NMR data to permit image reconstruction in any of three orthogonal planes of the object. The primary planes are generally referred to as the coronal, axial, and sagittal planes. The NMR data acquisition process is, however, not limited to these planes but is capable of acquiring data from oblique planes as well. With such multiplanar data-acquisition capability, it is desirable to test system operation in at least some representative orientations (e.g., coronal, sagittal, and axial planes). If performance is satisfactory for these, it can then be assumed that the system will operate satisfactorily in other orientations. This should be contrasted to CT where imaging information is acquired by measuring x-ray attenuation through the object slice of interest in a single transverse plane coincident with the plane of the x-ray beam. In CT, only if several contiguous planes are scanned, by advancing the object through the x-ray beam, can images corresponding to other orientations be calculated indirectly. In CT, therefore, there is no need for multiplanar system test capability.
Thus, it is apparent that a need exists in NMR for a phantom having the capability to provide multiplanar test data regarding NMR system operation without requiring the phantom to be repositioned, and with ease and low cost as described hereinbefore. The phantom should also provide a multi-parameter testing capability. It is, therefore, a principal object of the present invention to provide such a phantom.